An improved passive shimming method for low-field superconducting MRI

In magnetic resonance imaging (MRI), whether it is low-field or high-field MRI equipment, the homogeneity of the magnetic field is crucial for obtaining high-quality images. Low-Field MRI equipment typically uses a lower magnetic field strength (e.g., 0.2T to 0.5T). In this lower magnetic field, relatively minor magnetic field deviations can induce significant inhomogeneity in the primary magnetic field, which significantly impacts image quality and makes passive shimming more difficult. This article proposes some improvements to the existing passive shimming method, and the shimming process is divided into two stages: coarse shimming and fine shimming. In the process of fine shimming, a type of hollow core shimming piece is used, and a shimming method based on a minimum adjustment number of shimming pieces is also proposed. Finally, we practiced the improved method on a 0.55T low-field MRI system, improving the inhomogeneity from 1244.7 ppm of the traditional method to 8.3 ppm. In contrast, the original method could not find a new solution better than 10.7 ppm.